diff --git a/clang/lib/Basic/Module.cpp b/clang/lib/Basic/Module.cpp
index 0fd9c1dca3998..7879a11179b6d 100644
--- a/clang/lib/Basic/Module.cpp
+++ b/clang/lib/Basic/Module.cpp
@@ -301,7 +301,8 @@ bool Module::directlyUses(const Module *Requested) {
 
   // Anyone is allowed to use our builtin stdarg.h and stddef.h and their
   // accompanying modules.
-  if (!Requested->Parent && (Requested->Name == "_Builtin_stdarg" || Requested->Name == "_Builtin_stddef"))
+  if (Requested->getTopLevelModuleName() == "_Builtin_stdarg" ||
+      Requested->getTopLevelModuleName() == "_Builtin_stddef")
     return true;
 
   if (NoUndeclaredIncludes)
diff --git a/clang/test/Modules/Inputs/System/usr/include/inttypes.h b/clang/test/Modules/Inputs/System/usr/include/inttypes.h
deleted file mode 100644
index e69de29bb2d1d..0000000000000
diff --git a/clang/test/Modules/Inputs/System/usr/include/math.h b/clang/test/Modules/Inputs/System/usr/include/math.h
deleted file mode 100644
index e69de29bb2d1d..0000000000000
diff --git a/clang/test/Modules/Inputs/System/usr/include/module.map b/clang/test/Modules/Inputs/System/usr/include/module.map
index 5a15c70a394d2..1d88ca380f49a 100644
--- a/clang/test/Modules/Inputs/System/usr/include/module.map
+++ b/clang/test/Modules/Inputs/System/usr/include/module.map
@@ -1,24 +1,9 @@
 module cstd [system] {
-  // Only in system headers directory
-  module complex {
-    header "complex.h"
-  }
-
   // Only in compiler support directory
   module float_constants {
     header "float.h"
   }
 
-  // In both directories (compiler support version wins, forwards)
-  module inttypes {
-    header "inttypes.h"
-  }
-
-  // Only in system headers directory
-  module math {
-    header "math.h"
-  }
-
   // Only in system headers directory
   module stdio {
     header "stdio.h"
diff --git a/clang/test/Modules/Inputs/System/usr/include/stdint.h b/clang/test/Modules/Inputs/System/usr/include/stdint.h
index 209d54cd411ad..e8e50f90290ce 100644
--- a/clang/test/Modules/Inputs/System/usr/include/stdint.h
+++ b/clang/test/Modules/Inputs/System/usr/include/stdint.h
@@ -1,36 +1 @@
-#ifndef STDINT_H
-#define STDINT_H
-
 typedef int my_awesome_nonstandard_integer_type;
-
-// types needed by stdatomic.h
-
-typedef char int_least8_t;
-typedef short int_least16_t;
-typedef int int_least32_t;
-typedef long long int int_least64_t;
-typedef unsigned char uint_least8_t;
-typedef unsigned short uint_least16_t;
-typedef unsigned int uint_least32_t;
-typedef unsigned long long uint_least64_t;
-
-typedef char int_fast8_t;
-typedef short int_fast16_t;
-typedef int int_fast32_t;
-typedef long long int int_fast64_t;
-typedef unsigned char uint_fast8_t;
-typedef unsigned short uint_fast16_t;
-typedef unsigned int uint_fast32_t;
-typedef unsigned long long uint_fast64_t;
-
-typedef int intptr_t;
-typedef unsigned int uintptr_t;
-typedef int intmax_t;
-typedef unsigned int uintmax_t;
-
-// additional types for unwind.h
-
-typedef unsigned int uint32_t;
-typedef unsigned long long uint64_t;
-
-#endif /* STDINT_H */
diff --git a/clang/test/Modules/Inputs/builtin-headers/builtin-modules.modulemap b/clang/test/Modules/Inputs/builtin-headers/builtin-modules.modulemap
new file mode 100644
index 0000000000000..0fbb454dbb022
--- /dev/null
+++ b/clang/test/Modules/Inputs/builtin-headers/builtin-modules.modulemap
@@ -0,0 +1,29 @@
+module c_complex [system] {
+  header "complex.h"
+  export *
+}
+
+module c_float [system] {
+  header "float.h"
+  export *
+}
+
+module c_inttypes [system] {
+  header "inttypes.h"
+  export *
+}
+
+module c_limits [system] {
+  header "limits.h"
+  export *
+}
+
+module c_math [system] {
+  header "math.h"
+  export *
+}
+
+module c_stdint [system] {
+  header "stdint.h"
+  export *
+}
diff --git a/clang/test/Modules/Inputs/builtin-headers/c++/module.modulemap b/clang/test/Modules/Inputs/builtin-headers/c++/module.modulemap
new file mode 100644
index 0000000000000..c969f067a234f
--- /dev/null
+++ b/clang/test/Modules/Inputs/builtin-headers/c++/module.modulemap
@@ -0,0 +1,4 @@
+module cpp_stdint [system] {
+  header "stdint.h"
+  export *
+}
diff --git a/clang/test/Modules/Inputs/builtin-headers/c++/stdint.h b/clang/test/Modules/Inputs/builtin-headers/c++/stdint.h
new file mode 100644
index 0000000000000..22cebacfdb64e
--- /dev/null
+++ b/clang/test/Modules/Inputs/builtin-headers/c++/stdint.h
@@ -0,0 +1,6 @@
+#ifndef CPP_STDINT_H
+#define CPP_STDINT_H
+
+#include_next <stdint.h>
+
+#endif
diff --git a/clang/test/Modules/Inputs/builtin-headers/complex.h b/clang/test/Modules/Inputs/builtin-headers/complex.h
new file mode 100644
index 0000000000000..b6ef3c5a35b45
--- /dev/null
+++ b/clang/test/Modules/Inputs/builtin-headers/complex.h
@@ -0,0 +1 @@
+// Required by tgmath.h
diff --git a/clang/test/Modules/Inputs/System/usr/include/complex.h b/clang/test/Modules/Inputs/builtin-headers/float.h
similarity index 100%
rename from clang/test/Modules/Inputs/System/usr/include/complex.h
rename to clang/test/Modules/Inputs/builtin-headers/float.h
diff --git a/clang/test/Modules/Inputs/builtin-headers/inttypes.h b/clang/test/Modules/Inputs/builtin-headers/inttypes.h
new file mode 100644
index 0000000000000..ddb6bb83b5723
--- /dev/null
+++ b/clang/test/Modules/Inputs/builtin-headers/inttypes.h
@@ -0,0 +1,12 @@
+#ifndef INTTYPES_H
+#define INTTYPES_H
+
+// This creates an include cycle when inttypes.h and stdint.h
+// are both part of the cstd module. This include will resolve
+// to the C++ stdint.h, which will #include_next eventually to
+// the stdint.h in this directory, and thus create the cycle
+// cstd (inttypes.h) -> cpp_stdint (stdint.h) -> cstd (stdint.h).
+// This cycle is worked around by cstd using [no_undeclared_includes].
+#include <stdint.h>
+
+#endif
diff --git a/clang/test/Modules/Inputs/builtin-headers/limits.h b/clang/test/Modules/Inputs/builtin-headers/limits.h
new file mode 100644
index 0000000000000..8b137891791fe
--- /dev/null
+++ b/clang/test/Modules/Inputs/builtin-headers/limits.h
@@ -0,0 +1 @@
+
diff --git a/clang/test/Modules/Inputs/builtin-headers/math.h b/clang/test/Modules/Inputs/builtin-headers/math.h
new file mode 100644
index 0000000000000..b6ef3c5a35b45
--- /dev/null
+++ b/clang/test/Modules/Inputs/builtin-headers/math.h
@@ -0,0 +1 @@
+// Required by tgmath.h
diff --git a/clang/test/Modules/Inputs/builtin-headers/stdint.h b/clang/test/Modules/Inputs/builtin-headers/stdint.h
new file mode 100644
index 0000000000000..8f233fd7f45d6
--- /dev/null
+++ b/clang/test/Modules/Inputs/builtin-headers/stdint.h
@@ -0,0 +1,34 @@
+#ifndef STDINT_H
+#define STDINT_H
+
+// types needed by stdatomic.h
+
+typedef char int_least8_t;
+typedef short int_least16_t;
+typedef int int_least32_t;
+typedef long long int int_least64_t;
+typedef unsigned char uint_least8_t;
+typedef unsigned short uint_least16_t;
+typedef unsigned int uint_least32_t;
+typedef unsigned long long uint_least64_t;
+
+typedef char int_fast8_t;
+typedef short int_fast16_t;
+typedef int int_fast32_t;
+typedef long long int int_fast64_t;
+typedef unsigned char uint_fast8_t;
+typedef unsigned short uint_fast16_t;
+typedef unsigned int uint_fast32_t;
+typedef unsigned long long uint_fast64_t;
+
+typedef int intptr_t;
+typedef unsigned int uintptr_t;
+typedef int intmax_t;
+typedef unsigned int uintmax_t;
+
+// additional types for unwind.h
+
+typedef unsigned int uint32_t;
+typedef unsigned long long uint64_t;
+
+#endif
diff --git a/clang/test/Modules/Inputs/builtin-headers/system-modules.modulemap b/clang/test/Modules/Inputs/builtin-headers/system-modules.modulemap
new file mode 100644
index 0000000000000..0161ff80fe618
--- /dev/null
+++ b/clang/test/Modules/Inputs/builtin-headers/system-modules.modulemap
@@ -0,0 +1,71 @@
+module cstd [system] [no_undeclared_includes] {
+  module complex {
+    header "complex.h"
+    export *
+  }
+
+  module float {
+    header "float.h"
+    export *
+  }
+
+  module inttypes {
+    header "inttypes.h"
+    export *
+  }
+
+  module iso646 {
+    header "iso646.h"
+    export *
+  }
+
+  module limits {
+    header "limits.h"
+    export *
+  }
+
+  module math {
+    header "math.h"
+    export *
+  }
+
+  module stdalign {
+    header "stdalign.h"
+    export *
+  }
+
+  module stdarg {
+    header "stdarg.h"
+    export *
+  }
+
+  module stdatomic {
+    header "stdatomic.h"
+    export *
+  }
+
+  module stdbool {
+    header "stdbool.h"
+    export *
+  }
+
+  module stddef {
+    header "stddef.h"
+    export *
+  }
+
+  module stdint {
+    header "stdint.h"
+    export *
+  }
+
+  module tgmath {
+    header "tgmath.h"
+    export *
+  }
+
+  module unwind {
+    header "unwind.h"
+    export *
+  }
+}
diff --git a/clang/test/Modules/builtin-headers.mm b/clang/test/Modules/builtin-headers.mm
new file mode 100644
index 0000000000000..4c9ddec4e99c2
--- /dev/null
+++ b/clang/test/Modules/builtin-headers.mm
@@ -0,0 +1,41 @@
+// RUN: rm -rf %t
+// RUN: %clang_cc1 -cxx-isystem %S/Inputs/builtin-headers/c++ -internal-isystem %S/Inputs/builtin-headers -fsyntax-only -fmodules -fmodules-cache-path=%t -fmodule-map-file=%S/Inputs/builtin-headers/c++/module.modulemap -fmodule-map-file=%resource_dir/module.modulemap -fmodule-map-file=%S/Inputs/builtin-headers/system-modules.modulemap -fbuiltin-headers-in-system-modules -DSYSTEM_MODULES %s -verify
+// RUN: rm -rf %t
+// RUN: %clang_cc1 -cxx-isystem %S/Inputs/builtin-headers/c++ -internal-isystem %S/Inputs/builtin-headers -fsyntax-only -fmodules -fmodules-cache-path=%t -fmodule-map-file=%S/Inputs/builtin-headers/c++/module.modulemap -fmodule-map-file=%resource_dir/module.modulemap -fmodule-map-file=%S/Inputs/builtin-headers/builtin-modules.modulemap %s -verify
+
+// expected-no-diagnostics
+
+@import cpp_stdint;
+
+// The builtin modules are always available, though they're mostly
+// empty if -fbuiltin-headers-in-system-modules is used.
+@import _Builtin_float;
+@import _Builtin_inttypes;
+@import _Builtin_iso646;
+@import _Builtin_limits;
+@import _Builtin_stdalign;
+@import _Builtin_stdarg;
+@import _Builtin_stdatomic;
+@import _Builtin_stdbool;
+@import _Builtin_stddef;
+@import _Builtin_stdint;
+@import _Builtin_stdnoreturn;
+@import _Builtin_tgmath;
+@import _Builtin_unwind;
+
+#ifdef SYSTEM_MODULES
+// system-modules.modulemap uses the "mega module" style with
+// -fbuiltin-headers-in-system-modules, and its modules cover
+// the clang builtin headers.
+@import cstd;
+#else
+// builtin-modules.modulemap uses top level modules for each
+// of its headers, which allows interleaving with the builtin
+// modules and libc++ modules.
+@import c_complex;
+@import c_float;
+@import c_inttypes;
+@import c_limits;
+@import c_math;
+@import c_stdint;
+#endif
diff --git a/clang/test/Modules/compiler_builtins.m b/clang/test/Modules/compiler_builtins.m
index 8eeb65daa9e88..a5e6315cedc8e 100644
--- a/clang/test/Modules/compiler_builtins.m
+++ b/clang/test/Modules/compiler_builtins.m
@@ -1,7 +1,7 @@
 // RUN: rm -rf %t
-// RUN: %clang_cc1 -fsyntax-only -fmodules -fimplicit-module-maps -fmodules-cache-path=%t %s -internal-isystem %S/Inputs/System/usr/include -verify
-// RUN: %clang_cc1 -fsyntax-only -std=c99 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t %s -internal-isystem %S/Inputs/System/usr/include -verify
-// RUN: %clang_cc1 -fsyntax-only -fmodules -fmodule-map-file=%resource_dir/module.modulemap -fmodules-cache-path=%t %s -I%S/Inputs/System/usr/include -DNO_SYSTEM_MODULES -verify
+// RUN: %clang_cc1 -fsyntax-only -fmodules -fimplicit-module-maps -fmodules-cache-path=%t %s -I%S/Inputs/System/usr/include -verify
+// RUN: %clang_cc1 -fsyntax-only -std=c99 -fmodules -fimplicit-module-maps -fmodules-cache-path=%t %s -I%S/Inputs/System/usr/include -verify
+// RUN: %clang_cc1 -fsyntax-only -fmodules -fmodule-map-file=%resource_dir/module.modulemap -fmodules-cache-path=%t %s -I%S/Inputs/System/usr/include -verify
 // expected-no-diagnostics
 
 #ifdef __SSE__
@@ -11,19 +11,3 @@
 #ifdef __AVX2__
 @import _Builtin_intrinsics.intel.avx2;
 #endif
-
-#ifndef NO_SYSTEM_MODULES
-@import _Builtin_float;
-@import _Builtin_inttypes;
-@import _Builtin_iso646;
-@import _Builtin_limits;
-@import _Builtin_stdalign;
-@import _Builtin_stdarg;
-@import _Builtin_stdatomic;
-@import _Builtin_stdbool;
-@import _Builtin_stddef;
-@import _Builtin_stdint;
-@import _Builtin_stdnoreturn;
-@import _Builtin_tgmath;
-@import _Builtin_unwind;
-#endif